As the development of material with variable refractive index progresses, measuring the refractive index of a sample in 3-D becomes more desirable. For some optical samples, the refractive index does not remain constant throughout the thickness of the sample. For example, a gradient index (GRIN) lens may be made from materials with refractive indexes that vary throughout the thickness of the lens. Additionally, there are active programs to develop a material with writable index. A sample material with writable index can be made, for instance, by selectively exposing locations of the sample to laser light, and then heat-treating the entire sample. Exposing to laser light causes deposition of nanocrystals of a higher-index phase in the exposed locations. The initially formed nanocrystals may be annealed into larger grains of overall higher refractive index than the unexposed material. Lengthier exposure times lead to greater refractive index changes (increases).
Using focusing optics, regions of the sample can be exposed in close proximity to other regions that remain unexposed, allowing the possibility of writing what can later become index features in the finished sample. The process is nonlinear, enabling light that is projected into a focus to expose a region at a certain depth in the material (e.g., at the high-intensity focus of the light), while leaving material both at lesser and at greater depths relatively unexposed. This results in producing an isolated high refractive index island in 3-D, a process that can be replicated throughout the volume of the sample. Thus, leveraging this method, it is possible to write refractive-index features in full 3-D.